1. Field of the Invention
The present invention pertains to a method and apparatus for processing a video signal, and more particularly to removing (defeating) effects of copy protection signals from a video signal.
2. Description of the Prior Art
U.S. Pat. No. 4,631,603 ('603) by Ryan, issued on Dec. 23, 1986 entitled METHOD AND APPARATUS FOR PROCESSING A VIDEO SIGNAL SO AS TO PROHIBIT THE MAKING OF ACCEPTABLE VIDEO TAPE RECORDINGS THEREOF describes a video signal that is modified so that a television receiver will still provide a normal color picture from the modified video signal while a videotape recording of the modified video signal produces generally unacceptable pictures and is incorporated by reference.
The invention relies on the fact that typical videocassette recorder's automatic gain control systems cannot distinguish between the normal sync pulses (including equalizing or broad pulses) of a conventional video signal and added pseudo-sync pulses. Pseudo-sync pulses are defined here as pulses which extend down to a normal sync tip level and which have a duration of a least 0.5 microseconds. A plurality of such pseudo-sync pulses is added to the conventional video during the vertical blanking interval, and each of such pseudo-sync pulses is followed by a positive pulse of suitable amplitude and duration. As a result, the automatic gain control system in a videotape recorder will make a fake measurement of video level which causes an improper recording of the video signal. The result is unacceptable picture quality during playback. This approach utilizes positive-going pulses which occur at a time not ordinarily used for black level restoration in typical television receivers, and therefore does not give rise to black level depression in such typical television sets.
U.S. Pat. No. 4,819,098 ('098) by Ryan, issued on Apr. 4, 1989 entitled METHOD AND APPARATUS FOR CLUSTERING MODIFICATIONS MADE TO A VIDEO SIGNAL TO INHIBIT THE MAKING OF ACCEPTABLE VIDEOTAPE RECORDING describes modification of a television monitor receiver still produces a normal picture from the modified signal, whereas a videotape recording of this signal produces generally unacceptable pictures and is incorporated by reference. Videotape recorders have an automatic gain control circuit which measures the sync level in a video signal and develops a gain correction for keeping the video level applied to an FM modulator in the videotape recording system at a fixed, predetermined value. A plurality of positive pulses are added to a video signal with each immediately following a respective trailing edge of a normally occurring sync pulse. These added pulses are clustered at the vertical blanking interval of each field to minimize the affect of the same on the viewability of the picture defined by the signal while still causing the automatic level control circuit in a recorder to asses the video level at many times its actual value. The sync pulses themselves can also be at a reduced level, in order to enhance the effectiveness of the process.
U.S. Pat. No. 4,695,901 ('901) by Ryan, issued on Oct. 2, 1990 entitled METHOD AND APPARATUS FOR REMOVING PSEUDO-SYNC AND/OR AN AGC PULSES FROM A VIDEO SIGNAL describes removing pseudo-sync pulses and AGC pulses that have been added to a video signal to enable acceptable video recording thereof and is hereby incorporated by reference. The added signals previously interfered with acceptable video recording of the video signal because the automatic gain control of videotape recorders sensed false recording levels, while conventional television receivers were unaffected by those modifications to the video sign. Removal of the added pulses permits acceptable video recording of the previously modified video signal. A selectively-operable clipping circuit is used to remove selected negative-value components (i.e. pseudo-sync pulses)from the video signal, while added AGC pulses are effectively blanked from the video signal with an electrically-operated switch. Both the blanking and clipping functions are selectively achieved by sensing both the normal sync pulses of the video signal and the added pseudo-sync pulses. Method and apparatus are disclosed for "cleaning up" video signals modified by either the pseudo-sync pulses alone, the AGC pulses alone, or combinations thereof.
U.S. Pat. No. 4,336,554 ('554) by Okada et al., issued on Jan. 21, 1992 entitled CODE SIGNAL BLANKING APPARATUS (incorporated by reference) describes a code signal blanking apparatus comprising a switching means operative during a given period of a vertical blanking period of a television signal and a reference level setting means for producing an output of the reference level during said given period when the switching circuit is operative. A code signal is blanked by keeping a video signal level of the television signal at the vertical blanking period of the television signal.
U.S. Pat. No. 5,194,965 ('965) by Quan et al., issued on Mar. 16, 1993 entitled METHOD AND APPARATUS FOR DISABLING ANTI-COPY PROTECTION SYSTEM IN VIDEO SIGNALS describes a method and apparatus for disabling the effect of copy-protection signals placed in a recording video signals which is based on differences in the characteristics of television and VCR circuitry and is hereby incorporated by reference. Copy-protect signals include pseudo-sync pulses and/or added AGC pulses in the blanking interval of a video signal. The specific method described includes altering the level of the video signal during the vertical blanking interval, e.g. level-shifting, so as to render the copy-protect signals ineffective to prevent unauthorized copying by a VCR. A circuit for achieving the method includes a sync separator for detecting the vertical blanking interval, pulse generating circuits for producing pulses of predetermined widths during the interval, and a summing circuit for summing the predetermined pulses with copy-protect signals thereby to shift their level. An alternative method includes increasing the effective frequency of the copy-protect signals during the vertical blanking interval so as to achieve attenuation and/or low-pass filtering in the VCR circuitry to thereby render the signals ineffective in preventing copying. A circuit for achieving this method includes pulse narrowing and/or pulse multiplication circuitry which effectively increases the high-frequency content of the pseudo-sync and/or AGC pulses.
U.S. Pat. No. 5,157,510 ('510) by Quan et al., issued on Oct. 20, 1992 entitled METHOD AND APPARATUS FOR DISABLING ANTI-COPY PROTECTION SYSTEM IN VIDEO SIGNALS USING PULSE NARROWING (incorporated by reference) describes method and apparatus for disabling the effects of copy-protect signals added to a video signal using differences in the characteristics of television and VCR circuitry. Copy-protect signals as described include pseudo-sync pulses and/or added AGC pulses in the vertical blanking intervals of a video signal. The specific method described includes increasing the effective frequency of the copy-protect signals during the vertical blanking intervals so as to achieve attenuation and/or low pass filtering in the VCR circuitry to thereby render the signals ineffective in preventing copying. A circuit for achieving this method comprises pulse narrowing and/or pulse multiplication circuitry which effectively increases the high-frequency content of the pseudo-sync and/or AGC pulses.
U.S. patent application Ser. No. 08/433,283 ('283) entitled "VIDEO COPY PROTECTION PROCESS ENHANCEMENT AND VERTICAL PICTURE DISTORTIONS," by Quan filed on May 2, 1995 which is incorporated by reference, discloses defeating the affects of the signal generated by the '603 patent by modifying the relative amplitudes of the pseudo sync pulses to the normal sync pulse so that synchronizing pulse detection system within a recorder will only detect the normal synchronizing pulses, thus permitting normal recording of said copy protection signal.
U.S. patent application Ser. No. 08/435,575 ('575) entitled "VIDEO COPY PROTECTION PROCESS ENHANCEMENT AND VERTICAL PICTURE DISTORTIONS," by Quan filed on May 5, 1995 which is incorporated by reference, discloses various defeat mechanisms the affects of the signal generated by the '603 patent to permit normal recording of said copy protection signal.
U.S. Pat. No. 4,907,093 ('093) entitled "Method and Apparatus for Preventing The Copying of a Video Program," which is incorporated by reference, discloses a method and apparatus for detecting the ordered pairs of pseudo-sync pulses and white pulses described in the '603 patent and disabling the recording function of a video cassette recorder. The '093 patent discloses several detection methods.
U.S. Pat. No. 4,577,216, "Method and Apparatus For Processing a Video Signal," John O. Ryan, issued Mar. 18, 1986 and incorporated by reference, discloses modifying a color video signal to inhibit the making of acceptable video recordings thereof. A conventional television receiver produces a normal color picture from the modified signal. However, the resultant color picture from a subsequent video tape recording shows variations in the color fidelity that appear as bands or stripes of color error. Colloquially the modifications are called the "color stripe system" or the "color stripe process". Commercial embodiments of the teachings of this patent typically limit the number of video lines per field having the induced color error or color stripes.
Color video signals (both in the NTSC and PAL TV systems) include what is called a color burst. The color stripe system modifies the color burst. The suppression of the color subcarrier signal at the TV transmitter requires that the color TV receiver include (in NTSC) a 3.58 MHz oscillator which is used during demodulation to reinsert the color subcarrier signal and restore the color signal to its original form. Both the frequency and phase of this reinserted subcarrier signal are critical for color reproduction. Therefore, it is necessary to synchronize the color TV receiver's local 3.58 MHz oscillator so that its frequency and phase are in step with the subcarrier signal at the transmitter.
This synchronization is accomplished by transmitting a small sample of the transmitter's 3.58 MHz subcarrier signal during the back porch interval of the horizontal blanking pulse. The horizontal sync pulse, the front porch and blanking interval duration are essentially the same as that for black and white TV. However, during color TV transmission (both broadcast and cable) 8 to 10 cycles of the 3.58 MHz subcarrier that is to be used as the color sync signal are superimposed on the back porch. This color sync signal is referred to as the "color burst" or "burst". The color burst peak-to-peak amplitude is normally the same amplitude as the horizontal sync pulse.
In one commercial embodiment of the color stripe process, no color burst phase (stripe) modification appears in the video lines that have a color burst signal during the vertical blanking interval. These are lines 10 to 21 in an NTSC signal and corresponding lines in a PAL signal. The color stripe modifications occur in bands of four to five video lines of the viewable TV field followed by bands of eight to ten video lines without the color stripe modulation. The location of the bands is fixed ("stationary") field-to-field. This color stripe process has been found to be quite effective for cable television, especially when combined with the teachings of U.S. Pat. No. 4,631,603 also invented by John O. Ryan and incorporated herein by reference.
In NTSC TV, the start of color burst is defined by the zero-crossing (positive or negative slope) that precedes the first half cycle of subcarrier (color burst) that is 50% or greater of the color burst amplitude. It is to be understood that the color stripe process shifts the phase of the color burst cycles relative to their nominal (correct) position.
Further, the amount of phase shift in the color stripe process can vary from e.g. 20.degree. to 180.degree.; the more phase shift, the greater the visual effect in terms of color shift. In a color stripe process for PAL TV, a somewhat greater phase shift (e.g. 40.degree. to 180.degree.) is used to be effective.
Other variations of the Colorstripe.TM. process are described in the following provisional applications: Ser. No. 60/010015 entitled AN IMPROVED METHOD AND APPARATUS FOR MODIFYING THE COLOR BURST TO PROHIBIT VIDEO TAPE RECORDING by William J. Wrobleski, Ser. No. 60/010779 by William J. Wrobleski, entitled AN ADVANCED COLOR BURST METHOD AND APPARATUS FOR MODIFYING THE COLOR BURST TO PROHIBIT VIDEOTAPE RECORDING, Ser. No. 60/014246 by William J. Wrobleski, entitled A METHOD AND SYSTEM OF COPY PROTECTION USING AN ADVANCED AND SPLIT COLOR BURST (all incorporated by reference).
Methods and apparatuses for defeating the effects of the color burst modifications of the '216 patent and others are described in U.S. Pat. No. 4,626,890 entitled "Method and Apparatus for Removing Phase Modulation From the Color Burst," (incorporated by reference). Methods and apparatuses for detecting a color stripe signal and for additional defeat methods and apparatuses are described in application Ser. No. 08/438,155, filed May 9, 1995 entitled METHOD AND APPARATUS FOR DEFEATING EFFECTS OF COLOR BURST MODIFICATIONS TO A VIDEO SIGNAL by Ronald Quan and John O. Ryan. Application Ser. No. 60/010015, 60/0010779, 60/014246 and 08/438,155 ('155)are incorporated by reference.
U.S. Pat. No. 4,626,890, "Method and Apparatus For Removing Phase Modulation From the Color Burst", John O. Ryan, issued Dec. 2, 1986 and incorporated by reference, discloses removing the phase modulation of the U.S. Pat. No. 4,577,216. This removal is useful in eliminating much of the effects of the process disclosed in U.S. Pat. No. 4,577,216 for recording.
The '155 application by Quan et al. describes a color stripe process for preventing recording of video signals, the color burst present on each line of active video is modified so that any subsequent video tape recording of the video signal shows variations in the color fidelity that appear as undesirable bands or stripes of color error. This color stripe process is defeated first by determining the location of the video lines including the color stripe process, either by prior experimentation or by on-line detection. Then some or all of the lines including the modified color bursts are modified so as to render the overall video signal recordable. The modification is accomplished in a number of ways, including phase shifting the color stripe burst into the correct phase, replacing some of the color stripe bursts or a portion of particular color stripe bursts so that they are no longer effective, and mixing the color stripe burst with color stripe signals of the correct phase so as to eliminate most or all of the phase error present. The modified color bursts are defeated, in other versions, by modifying the horizontal sync pulse signals immediately preceding the modified color bursts so that the modified color bursts are not detected by a VCR and hence have no effect.
All of the defeat methods described in the referenced defeat patents use analog techniques. With the advent of greater use of digital technology, there is a need for digital implementations of these techniques.